No, I don't really question others' capacity to choose a partner. I guess I don't really consider myself a social connoisseur enough to judge that. I mean, obviously some relationships don't work out in the end, and it's often due to a character flaw of one or both parties, but how can someone predict ahead of time what character deficits their partner might show in the future? I guess, yeah, sometimes it's obvious from the beginning, but people are attracted to what they're attracted to. It's easy to be "objective" about someone when you're disimpassioned. If my heart were really lit on fire by someone, maybe I'd be "irrational" about them too!
As for getting a partner, that's a completely different question. I don't "question" it, but yeah, it's obvious that some people--especially some males--have a hard time getting a partner. That's just an objective fact. Like, if it's been 5 years that you've desperately wanted a partner and haven't gotten one, I guess it stands to reason that you have little "capacity" to get a partner, for whatever reason. I mean, I guess you could say it's a always function of how high (or low) they set their standards, but I can understand not wanting to compromise too much on standards.